Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHA) is a common completely automated program for determining whether a user is a computer or human. In the process of the CAPTCHA test, a server automatically generates a question to be answered by the user. The question generated by the server is solvable by human only, but not by the computer, thus the user who solves the question is determined as human.
However, an Optical Character Recognition (OCR) technology has been developed by skilled persons as technologies develop. The Optical Character Recognition technology is utilized to analyze an image that is generated from scanning a document, to obtain information such as text contained in the image. Particularly, characters in the document are scanned by a digital scanning device such as a scanner and a digital camera to generate an image, then shapes of the characters are determined through distinguishing bright pixels and dark pixels in the image, and then the shapes of the characters are converted to corresponding computer texts by the Optical Character Recognition technology. Through the OCR technology, it is therefore possible for a computer to answer the question generated by the server.
In order to prevent a hacker from recognizing CAPTCHA codes and submitting answers by technologies such as the OCR technology, there is another type of existing CAPTCHA which provides a CAPTCHA image which is formed by lines and irregular characters. In verifying a user, a CAPTCHA code in the form of an image is generated by a server according to a preset rule, sent to a terminal device, and displayed on a display interface of the terminal device; the user examines the CAPTCHA code, recognizes a character string comprised of digitals, English letters, Chinese characters or other characters contained in the image of the CAPTCHA code, and then fills in the recognized character string which is then submitted to the server for a user verification; and the server determines whether the submitted character string conforms to that contained in the image of the CAPTCHA code, and if the submitted character string conforms to that contained in the image, the user is determined as human, as illustrated in FIG. 1 which is a schematic view showing the CAPTCHA code in the prior art a conventional method.
However, the CAPTCHA code in the form of the image comprised of lines and irregular characters is defective in the prior art. Once enough samples are collected by hackers, such CAPTCHA code in the form of the image may be recognized automatically by a computer program which is trained by means of the OCR technology. Even if such CAPTCHA code is optimized and improved, the CAPTCHA code will still be recognized automatically by the computer program as long as the number of images from a background image library for the CAPTCHA code is limited. Therefore, the security of the existing CAPTCHA code in the form of the image is unsatisfying.
Additionally, in order to promote the capability against the automatic recognition made by the computer program, the difficulty in recognizing such CAPTCHA code in the form of an image is increased, but at the same time the recognition by human users is affected as well and hence the user experience is degraded. Though such CAPTCHA code is always shown with a hint of “click to change CAPTCHA code” which can be clicked to get a new CAPTCHA code, the user experience is still poor due to the high recognition difficulty.